Coatings based on thermoplastic resins have many good properties, such as exterior durability. Their solvent resistance, however, is very poor due to lack of cross-linking among the polymer molecules. They often also require large amounts of solvent to reduce viscosity to a level where the polymeric vehicle or formulated coating composition can be conveniently applied to a substrate.
Efforts have been made to develop low molecular weight thermosetting resins which have lower viscosities and high solids. These resins provide coating formulations which may be cross-linked to form films with improved solvent resistance. The most common thermosetting acrylic resins contain hydroxyl groups. These resins are cross-linked with melamine-formaldehyde resins or polyisocyanates and usually require a baking temperature of about 150.degree. C.; and hence, consume energy for their application and inconveniently require a heating step for a cure into a coating binder. Furthermore, isocyanates are sensitive to moisture, often inconveniently require prepolymers and are toxic.
Terminal acetylenes which have propargyl groups (--CH.sub.2 C.tbd.CH) are known to undergo an oxidative or Glaser coupling. Glaser, 154 Ann. 137 (1870); Glaser, 2 Ber. 422 (1869). This reaction is generally effected by air or oxygen either in an aqueous solution of a mixture of copper (I) chloride and ammonium chloride or in an organic solution of an amine complex of a copper (I) salt. Raphael, Acetylene Compounds in Organic Synthesis, 127, Academic Press, N.Y. (1955); Hay, Oxidative Coupling of Acetylenes, 25 J. Org. Chem. 1275 (1960); Hay, Oxidative Coupling of Acetylenes. II., 27 J. Org. Chem. 3320 (1962). The reaction proceeds smoothly at room temperature; and even though it has found a wide variety of applications in organic synthesis of conjugated acetylenes, it has not been used in connection with polymerization reactions or the coatings industry.
Despite the fact that propargyl polymers are not new in connection with polymers [See U.S. Pat. No. 3,562,231 to D'Alelio, the bromination of polymers of propargyl methacrylate to yield hydrolyrically stable materials; U.S. Pat. No. 4,226,800, to Picklesimer, a process for the preparation of phenolic materials containing propargyl groups where a polyhydric phenolic material is reacted with propargyl bromide; Wu et al., "Coatings Curable with Low-Emission Radiation", in Radiation Curing of Polymeric Materials, ACS Symposium Series 417, Washington, D.C. (1990), radiation and/or thermal curable materials based on linear polymers of propargyl derivatives of bisphenols], the advantages of acrylic polymers for coatings and polymeric vehicles have not been advantageously combined with the ability of acetylenic groups or propargyl groups to undergo oxidative coupling to provide polymeric vehicles for coating binders.
Indeed, oxidative coupling of acetylenic polymers according to the invention not only is an attractive approach for development of air drying resins for coating applications, it also provides a high solids polymeric vehicle to reduce the emissions of volatile organic compounds (VOCs) from such coating compositions.
An object of this invention is to provide a fast curing polymeric vehicle which will cure into a coating binder which is hard, has good impact resistance and is resistant to acidic and basic conditions.
Another object of this invention is to provide a fast curing air drying polymeric vehicle which will air dry at room temperature.
Further objects and advantages of the invention will be found by reference to the following description.